Miniature biosensors utilizing enzyme-containing optodes and electrodes for monitoring biochemicals often include mass-transport controlling membranes. The membranes can affect some or all of the characteristics of the optodes or electrodes, including their sensitivity, size, apparent stability, dynamic range and selectivity. Micro-membranes for use with miniature biosensors typically cannot be easily cut to size from pre-formed membranes and if cut by a precision tool, such as a laser or an electron beam, their placement on and attachment to the surface of an electrode or optode can be difficult.
The reproducible casting of micro-membranes can also be difficult. For cast micro-membranes, the pore sizes and their distribution are typically determined by the relative rates of nucleation and mass-transport during generation of the membrane by phase separation as a result of solvent evaporation. The outcome of the simultaneously occurring nucleation and the mass transport processes depends on the evolution (meaning the time-dependence during the solvent evaporation) of the viscosity, the concentrations of the solvent and the non-solvent, and the membrane's leached phase and its residual phase. These are affected by time-dependent temperature gradients and by the time-dependent gradient of the partial pressure of the evaporating solvent over a droplet, the dimensions of which shrink and are a function of the time-dependent contact angle with the wetted surface.